The present invention relates to improvements in lift truck-mounted clamping apparatus for picking up, transporting and stacking loads. More particularly the invention relates to improvements in the texturing of the load-engaging surfaces of the contact pads of a pair of openable and closeable clamp arms, such texturing being specifically designed to provide more efficient and damage-free operation in view of the manipulative procedures and fragile material characteristics peculiar to the handling of paper rolls, cardboard cartons and similar loads.
The roll engagement surfaces of lift truck paper roll clamp contact pads have conventionally comprised a roughened frictional surface of closely spaced, randomly oriented small protrusions defining irregularly shaped small depressions therebetween, formed on an elongate concave surface which curves about an axis which extends longitudinally of the surface and transversely to the lift truck so as to form a generally partial cylindrical surface. The randomly oriented, irregularly shaped fine protrusions and depressions of such conventional engagement surfaces provide frictional resistance to slippage between the contact pad and the paper roll which is equal in all directions.
With respect to lift truck carton clamps for handling loads contained in cardboard boxes and the like, the load-engaging surfaces of the contact pads have conventionally been of a flat configuration. One particularly relevant type of load-engaging surface previously utilized on this type of clamp is a resilient surface having longitudinal parallel grooves and lands formed therein as shown for example in Purpura U.S. Pat. No. 2,812,089. Inasmuch as carton clamps utilize much less clamping pressure than do paper roll clamps, the use of resilient load-engaging surfaces compensates for the relatively low clamping force by providing a high coefficient of friction to resist slippage of the loads from between the clamp arms. Purpura evidences a common belief that forming parallel grooves in the resilient load-engaging surfaces increases the resistance of the contact pads to slippage of smooth cardboard surfaces, typical of cartons and boxes, in directions of slip both parallel to and perpendicular to the grooves. While it is true that the resistance to slip of such grooved resilient contact surfaces is quite high in both directions of slip, it has been found that such resistance can be even higher if smooth resilient surfaces are utilized.
The specific procedures peculiar to lift truck paper roll and carton handling, together with the fragile nature of the rolls and cartons being handled, causes certain problems to result from the use of finely textured paper roll contact pad surfaces or resilient smooth or grooved carton clamp pad surfaces. The major problem stems from the fact that a lift truck engages a roll or stack of cartons by driving forwardly toward the roll to cause one or more of the clamp arms to "knife-in" between the side of the load and the surfaces of adjacent loads or, if a paper roll is lying horizontally rather than vertically, between the roll and the floor. In any case, the high frictional resistance to slippage provided by either finely textured conventional paper roll contact pad surfaces, or resilient carton clamp surfaces, present a disadvantage during the "knifing-in" movement because considerable frictional force between the contact surface and load can oppose such movement, causing abrasion or tearing of the relatively fragile paper or cardboard.
Moreover, both of the above-described conventional types of contact pad surfaces are highly susceptible to wear. In the case of finely textured paper roll contact pads, since the relatively fine protrusions have small bearing surfaces the depressions between them must be shallow to permit permanent indentation or cutting of the paper. Since the depth of the depressions (and accordingly the height of the protrusions) must be shallow, the texturing is highly susceptible to removal by wear from the surface, leaving a smooth metal contact pad surface having a significantly reduced capability for frictional resistance to slippage.
In the case of grooved resilient carton clamp pads, although the grooves are deeper, the resilient material is much more susceptible to wear than metal. Moreover, although wearing of a resilient grooved pad to a smooth configuration probably increases, rather than decreases, the frictional resistance of the pad to slippage, such increase in the already high frictional resistance increases the likelihood of load abrasion and damage referred to above during the "knifing-in" process.
While pads having smooth metal load-engaging surfaces have a much lower coefficient of friction than either of the above-identified types of clamp pad surfaces, and would therefore minimize the likelihood of load damage resulting from the "knifing-in" process, and are also not susceptible to any significant wear, the ability of such clamp pads to hold the weight of a load by frictional engagement thereof is also significantly reduced, with the substantial disadvantage that higher hydraulic clamping pressures would be required for handling the same loads.